Method of miscible flooding



OR licifilfis to'l United States Patent 3,203,481 METHGD OF MISCIBLE FLOODING John N. Dew and Philip W. Reed, Ponca City, Okla, as-

signors to Continental Oil Company, Ponca City, Okla, a corporation of Delaware No Drawing. Filed Sept. 1, 196i), Ser. No. 53,378 7 Claims. (Cl. 1669) The present invention pertains to the recovery of hydrocarbons from subterranean reservoirs by the utilization of miscible fluid flooding media. More particularly this invention relates to the secondary recovery of hydrocarbons by a method of combined flooding media which is novel and unique in the art, in order that substantially all of the oil in a reservoir may be recovered.

The secondary recovery of hydrocarbons generally rerfers to those recovery efforts which are applied in order to obtain production subsequent to the economic production obtainable by utilizing naturally-occurin g energy or conventional pumping means. The use of miscible fluids in secondary recovery operations is known in the art, particularly the injection of low molecular weight normally gaseous hydrocarbons. Methods utilizing such hydrocarbons are extremely successful with regard to the displacement efficiency of the hydrocarbons, particularly the ability to displace essentially all of the hydrocarbons from the formation; but the volumetric efliciency of this flooding media is a disadvantage. The hydrocarbons inherently finger and channel to a great extent, and a large portion of the reservoir is bypassed, though the volume of injected hydrocarbons may be great, which raises further disadvantages of expense per volume and inability to obtain suflicient volumes of the flooding medium.

The prior art teachings in the miscible flooding phases of the secondary recovery of oil disclose that it is well known to utilize low molecular weight hydrocarbons and more particularly to use these materials in conjunc- .tion with other more economic flooding media which are immiscible therewith, such as water or gas. The most recent trend in miscible flooding is to utilize slugs of the various miscible hydrocarbons with the initial injection of the flooding medium being followed by a series of miscible fluids having successively reduced viscosity which is normally referred to as grading back, even to the extent that the latter stages of .the flooding are accomplished by utilizing methane or inert gases. The above miscible fluids have also been utilizing inert gases such as nitrogen and combustion gas under conditions of non-miscibility with the liquid hydrocarbons to achieve a favorable mobility ratio in order to accomplish the optimum displacement and volumetric efficiencies, but all of the methods which have been disclosed and taught in the prior art have certain disadvantages.

An object of this invention is to provide a method of recovering hydrocarbons from a subterranean reservoir wherein the overall recovery results in the production of substantially all of the remaining oil in place.

Another object on? the present invention is to provide an improved method of secondary recovery wherein the expensive displacing media are recoverable.

Other objects and advantages of the present invention will be more fully disclosed in the errsuiang detailed description of the invention.

This invention broadly comprises injecting a volume of liquefied low molecular weight hydrocarbon followed by the injection of a liquefied low molecular weight hydrocarbon having a viscosity greater than that initially injected, and completing the recovery of the hydrocarbons in place by the injection of a volume of immiscible gaseous material having a lesser viscosity than the previsides in altering the established order of viscosity with,

the injected hydrocarbon displacing media, that is, varying the viscosities within the injected miscible hydrocarbons in an upward manner rather than the normal successive reduction of viscosities throughout the flooding program.

Prior to the initial injection phase of the present method the reservoir containing the hydrocarbons to be recovered should be subjected to conditions so as to establish pressures therein which are suflicient to maintain the subsequently injected displacing media in a liquefied form, that is, those media which are intended to be in liquefied form. This pressure may be naturally occuring, or it may be necessary to inject air, gas, or other material sufl'icient to establish the requisite pressures. These pressures are determined in accordance with the established and well known phase behavior characteristics of hydrocarbons and hydrocarbon mixtures. The amount of pressure necessary can be determined by means well known in the art for each individual reservoir.

The initial recovery step of this method is the injection of a volume of liquefied low molecular weight hydrocarbon having an intermediate viscosity range containing 2 to 6 carbon atoms, preferably propane, through the well bore by conventional means and into the reservoir. This flooding medium normally has a viscosity less than the hydrocarbon oi the reservoir which is being subjected to the recovery process and, being miscible with the hydrocarbons in place, has an extremely high displacement efliciency. It displaces substantially all of the hydrocarbons which it contacts from the well bore and surrounding formation, toward the recovery wells. The displacing medium, having lower viscosity than the hydrocarbon of the reservoir, has inherent volumetric inefliciencies and tends to bypass portions of the reservoir hydrocarbons, due to fingering and channeling; but these bypassed hydrocarbons are concurrently subjected to reduction in viscosity, due to mixing, mainly by molecular diffusion between the bypased hydrocarbon and the injected medium, The volume of the initial displacing hydrocarbon should be in the order of about 1 to 10 percent of the pore space of the reservoir, preferably about 4 percent of this pore space. The actual amount which will be required for each formation must be determined for each individual reservoir as governed by the oil sauration, the viscosity of the oil, and other known factors.

' The second phase of the present invention is the injection of another liquefied low molecular weight hydrocarbon containing 3 to 7 carbon atoms which is miscible with the hydrocarbon displacement medium injected in the initial phase, but which has a greater viscosity than the initially injected hydrocarbon, such as butane, when the initial fluid is propane.

The second displacing medium continues to displace hydrocarbons from the reservoir; and due to the greater viscosity, it is not subject to channeling and fingering to such an extent as the initially injected hydrocarbon medium. The secondary phase of the present invention tends to establish a more favorable mobility ratio, thereby increasing the volumetric efliciency while maintaining the substantially complete displacement efliciency of miscible displacement. The volume of the hydrocarbon injected in this secondary phase of the present invention should be in the order of about 1 /2 to 1-5 percent of the pore space of the formation, preferably about 5 percent, but .again dependent upon the individual reservoir characteristics. The injection of the second phase creates a mixture with the initial phase establishing a bank or butter zone wherein the mobility ratio is greatly improved, and thereby imparts a more uniform displacement of the hydrocarbons initially present in the reservoir as well as those injected in the first phase.

The ultimate phase of the method disclosed herein is the injection of an inexpensive displacement gas which is immiscible with the displacing medium injected during the sceond phase and has a viscosity greatly decreased [from the preceding displacement hydrocarbon and only slightly soluble therein. The economically preferable gaseous material for use in the ultimate phase is methane gas, but other gases such as combustion gas, carbon dioxide or nitrogen which are inert in nature are also satisfactory, provided suflicient volumes are available. The immi-scibility of the ultimate displacement gas with regard to the previously injected fluids causes the displacement flow controlled by relative permeability and capillary pressure relationships which cause some residual hydrocarbon to remain in the area entered by the dis placing immiscible gas. The mobility of the displacing vgas is less than if the displacement has been miscible, thereby permitting complete displacement of hydrocarbons due to the lower mobility of the displacing gas which promotes the high volumetric coverage. The hydrocarbons injected as displacement media into the reservoir formation are recovered by the displacing gas of the ultimate injection phase by conventional displacement or by vaporizing any bypassed media into this gas, thereby facilitating recovery at the production wells. This vaporization of the injected hydrocarbons occurs behind the front of the ultimate displacing gas so the eflective mobility at the Zone of contact between the hydrocarbon injected in the second phase and the displacing gas is not affected, thereby preserving the volumetric coverage.

The volume of the displacement gas which must be injected in the ultimate phase of the present invention is readily determinable in advance by methods well known in the art, but it is unnecessary to do so for purposes other than economic studies, in that the actual volume of the ultimate displacement gas can be determined by analyzing the production products at the recovery well.

A variation of the present invention is the injection of a displacement medium in the ultimate phase which is miscible with the initial displacement medium but immiscible with the second displacement medium. Thus as the ultimate medium penetrates or fingers through the second phase medium into the first pha se medium, the rate of penetration is reduced due to the miscibility of the first medium and the ultimate medium. As there is full miscibility between the reservoir oil and the first phase and between the first phase and the ultimate phase which contacts it, complete displacement of oil is assured as long as a distinct first phase is maintained. When using the common materials, butane, propane and methane, this variation can be accomplished by operating at known conditions of pressure and temperature controlling the miscibility between methane-butane and methane propane.

Another variation of the present invention which is adaptable to certain reservoirs is the injection of water with the hydrocarbons injected in the various phases of the present invention. This injection of volumes of displacing media and water, as determined by relative permeability and saturation change relationships to give approximately equal hydrocarbon and water velocity, can be accomplished in a simultaneous manner by conventional means, or water may be injected in a slug-wise manner wherein the water and displacing media are injected alternately in small amounts, due to the fact that the dispersion of the separately injected water and hydrocarbon is completely overcomc atter the two have moved a relatively short distance through the reservoir, achieving essentially simultaneous flow. This water injection may be commenced at any of the phases of the present invention, but upon commencing such operations it is necessary that they be maintained, that is, if the water is injected during the second phase, it is thereafter necessary to inject water in the third phase also. This water injection has certain economic advantages, due to the relatively inexpensiveness to the displacing medium per volume of formation filled, coupled with the fact that it is also possible to conclude the recovery process by the injection of solely a water phase, dependent upon the economics of the individual formation.

The above disclosed method results in the recovery of substantially all of the hydrocarbons within the formation; and upon analyzation of the product at the producing wells, it is readily apparent when these reservoir hydrocarbons have been recovered. At the time when it has been determined that the ultimate injection can be terminated, it is thereafter possible to recover substan tially all of the displacement hydrocarbons which have been previously injected into the formation. This may be accomplished by opening the recovery wells to a lower pressure and allowing the pressures within the formation to produce these displacement fluids at the recovery wells. Certain amounts of these fluids will be produced in the liquid phase; but upon reduction of the pressure of the torm ation, these liquefied low molecular weight hydrocarbons will gasify, and the expanded volume will flow as a vapor or gas phase in such a manner that these displacement fluids will be recoverable at the appropriate wells. This is not a necessary step in the recovery of the reservoir hydrocarbon, however, as a matter of economic advantage, it is quite preferable to subject the injected displacement fluids to the foregoing recovery efforts.

From the foregoing it will be apparent that the method of the present invention is susceptible to a variety of applications and embodiments, one of which is included below by way of an example.

Example A conventional five-spot wel'l pattern covering 10 acres traversing an oilsbearing sand formation having an average thickness of 7 feet at a depth of about 5043 feet is suitable for secondary recovery. The method of secondary recovery of the present invention could be applied to the portion of the reservoir encompassed Within the area of the pattern most readily by utilizing the center well as the injection well and the remaining four peripheral wells as production wells. The temperature of the reservoir was approximately 128 F. and the pressure was approximately 1275 psi, these conditions being suflicien't to maintain the initial displacing hydrocarbons in the liquid state.

Determinations by known methods indicate the overall volume of the formation would be 2,185,000 cubic feet and the pore volume would be 73,900 barrels and containing approximately 47,000 barrels of reservoir oil which is subject to recovery by the present method. For the initial injection of displacement medium 1092 barrels of plant propane product containing up to 15 percent ethane were injected through the injection well into the formation, followed by the injection of 1643 barrels of plant butane product during the second phase of the invention. The method was completed upon the injection of 32,200,000 standard cubic feet of essentially methane gas which resulted in the production of 39,600 barrels of the reservoir oil in place at the outset of the operation or 84.2 percent. It was economically desirable to produce the injected hydrocarbons, and upon release of the reservoir pressure an additional 620 barrels of the injected hydrocarbons were recovered.

It is to be understood that the present invention is not limited to the foregoing example but rather that the invention is defined by the appended claims.

We claim:

1. A method for the secondary recovery of hydrocarbons from a subterranean hydrocarbon-bearing reservoir traversed by an injectionwell and a reggveny well wh igh comprises injecting a gaseous fluid into said reservoir in sufficient volumes establishing pressure in excess of the vapor pressure of liquefied low molecular weight hydrocarbons to be utilized as displacement media, injecting a firstliquefied low molecular weight hydrocarbon containing from 2 to 6 carbon atoms down the injection well into the reservoir in an amount of about 1.0 to 10.0 percent of the hydrocarbon pore space thereof, said first hydrocarbon being miscible with the reservoir hydrocarbons, inje cting a s ecgn d liquefied low molecular weight hydrocarboncontaining ffifi fii to 7 carbon atoms down the injection well into the reservoir in an amount of about 1.5 to percent of the hydrocarbon pore space thereof, said second hydrocarbon being miscible with and having a greater viscosity than said first hydrocarbon, injecting a displacement gas down j1l i1lj6CtlOnJpl1 into the reser- "'vo'ir, saidg 'a s' bin g immiscible with and having a lesser viscosity than said second hydrocarbon, maintaining the reservoir pressure at a value greater than the normal vapor pressures of said first and second hydrocarbons under the conditions that prevail within the reservoir, and producing the reservoir hydrocarbons displaced from the reservoir through the recovery well.

2. A method for the secondary recovery of hydrocarbons from a subterraneans hydrocarbon-bearing reservoir traversed by an injection well and a recovery well which comprises injecting a gaseous fluid into said reservoir in sufficient volumes establishing pressure in excess of the vapor pressure of liquefied low molecular weight hydrocarbons to be utilized as displacement media, injecting a first liquefied low molecular Weight hydrocarbon containing from 2 to 6 carbon atoms down the injection well into the reservoir in an amount of about 1.0 to 10.0 percent of the hydrocarbon pore space thereof, said first hydrocarbon being miscible with the reservoir hydrocarbons, injecting a second liquefied low molecular weight hydrocarbon containing from 3 to 7 carbon atoms down the injection well into the reservoir in an amount of about 1.5 to 15 percent of the hydrocarbon pore space thereof, said second hydrocarbon being miscible with and having a greater viscosity than said first hydrocarbon, injecting a displacement gas down the injection well into the reservoir, said gas being immiscible with and having a lesser viscosity than said second hydrocarbon, maintaining the reservoir pressure at a value greater than the normal vapor pressures of said first and second hydrocarbons under the conditions that prevail Within the reservoir, displacing and producing the reservoir hydrocarbons from the reservoir through the recovery Well, and terminating injection of said displacement gas upon the recovery of substantially all of the reservoir hydrocarbons.

3. A method as defined in claim 1 in which water is injected simultaneously with the injection of the displacement gas.

4. A method as defined in claim 1 in which water is injected simultaneously with the injection of the second hydrocarbon and the displacement gas.

5. A method as defined in claim 1 in which water is injected simultaneously with the injection of the first and second hydrocarbons and the displacement gas.

6. A method as defined in claim 1 in which the amount of said first hydrocarbon is 4 percent of the hydrocarbon pore space of the reservoir and the amount of said second hydrocarbon is 5 percent of the hydrocarbon pore space of the reservoir.

7. A method as defined in claim 1 in which said first injected hydrocarbon is propane, said second injected hydrocarbon is butane, and said displacement gas is methane.

References Cited by the Examiner UNITED STATES PATENTS 2,669,307 2/54 Mulholland et a1. 1669 2,742,089 4/56 Morse et al 1669 2,771,139 11/56 Johnson et al. 1669 2,968,350 1/61 Slobod et a1 1669 FOREIGN PATENTS 696,524 9/53 Great Britain.

OTHER REFERENCES Gibbon, A., Injected Liquid Propane Boosts Oil Production, World Oil, May 1957, pp. 92-93.

Kieschnick, What Is Miscible Displacement, The Petroleum Engineer, August 1959.

CHARLES E. OCONNELL, Primary Examiner.

BENJAMIN BENDETT, Examiner. 

1. A METHOD FOR THE SECONDARY RECOVERY OF HYDROCARBONS FROM A SUBTERRANEAN HYDROCARBON-BEARING RESERVOIR TRAVERSED BY AN INJECTION WELL AND A RECOVERY WELL WHICH COMPRISES INJECTING A GASEOUS FLUID INTO SAID RESERVOIR IN SUFFICIENT VOLUMES ESTABLISHING PRESSURE IN EXCESS OF THE VAPOR PRESSURE OF LIQUEFIED LOW MOLECULAR WEIGHT HYDROCARBONS TO BE UTILIZED AS DISPLACEMENT MEDIA, INJECTING A FIRST LIQUEFIED LOW MOLECULAR WEIGHT HYDROCARBON CONTAINING FROM 2 TO 6 CARBON ATOMS DOWN THE INJECTION WELL INTO THE RESERVOIR IN AN AMOUNT OF ABOUT 1.0 TO 10.0 PERCENT OF THE HYDROCARBON PORE SPACE THEREOF, SAID FIRST HYDROCARBON BEING MISCIBLE WITH THE RESERVOIR HYDROCARBONS, INJECTING A SECOND LIQUEFIED LOW MOLECULAR WEIGHT HYDROCARBON CONTAINING FROM 3 TO 7 CARBON ATOMS DOWN THE INJECTION WELL INTO THE RESERVOIR IN AN AMOUNT OF ABOUT 1.5 TO 15 PERCENT OF THE HYDROCARBON PORE SPACE THEREOF, SAID SECOND HYDROCARBON BEING MISCIBLE WITH AND HAVING A GREATER VISCOSITY THAN SAID FIRST HYDROCARBON, INJECTING A DISPLACEMENT GAS DOWN THE INJECTION WELL INTO THE RESERVOIR, SAID GAS BEING IMMISCIBLE WITH AND HAVING A LESSER VISCOSITY THAN SAID SECOND HYDROCARBON, MAINTAINING THE RESERVOIR PRESSURE AT A VALUE GREATER THAN NORMAL VAPOR PRESSURES OF SAID FIRST AND SECOND HYDROCARBONS UNDER THE CONDITIONS THAT PREVAIL WITHIN THE RESERVOIR, AND PRODUCING THE RESERVOIR HYDROCARBONS DISPLACED FROM THE RESERVOIR THROUGH THE RECOVERY WELL. 